The present invention relates to packaging for protecting electronic components or assemblies from potentially damaging electrostatic discharges during shipment and storage.
Each of U.S. Pat. Nos. 4,154,344 and 4,156,751, both Yenni, Jr., et al, (both of which are hereby incorporated by reference) after discussing prior envelopes for protecting electronic components, discloses a material for such envelopes that provides better protection from electrostatic discharges while still permitting the envelope to be transparent. The Yenni, Jr., et al material comprises a electrically insulating, polymeric sheet having first and second major surfaces with the first major surface being exterior of the second major surface. On the inner surface of the envelope, an antistat provides a surface resistivity in the range of 10.sup.8 to 10.sup.14 ohms per square, and exterior of the electrically insulating polyermic sheet, an electrically conductive layer provides a surface resistivity of no greater than about 10.sup.4 ohms per square. When the conductive layer is of metal, it preferably is covered by a layer of abrasion resistant material which can be quite thin. Preferably, the envelope is formed by folding a single sheet and sealing its edges to ensure electrical continuity of both the antitatic and electricaly conductive layers. The polymeric sheet should have sufficiently high impedance, e.g., 10.sup.15 ohms-cm, to prevent the direct coupling of external electrostatic charges to the contents of the envelope.
While the container of each of the Yenni, Jr., et al patents does not hold a shape, that of U.S. Pat. No. 4,553,190, Mueller (which is hereby incorporated by reference) does. The layering of the wall of a preferred Mueller container is similar to that of a preferred Yenni, Jr., et al envelope except that its electrically insulating polymeric sheet is sufficiently thick to form a shape-retaining tube. The inner surface of Mueller's transparent wall may have an antistatic layer to help prevent electroststic charge buildup on the packaged electronic components such as could be caused by triboelectric charging, preferably one providing a surface resistivity of from 10.sup.7 to 10.sup.14 ohms per square. The Mueller material also has an electrically conductive layer which provides a surface resistivity of not more than 10.sup.6 ohms per square. The electrically conductive layer preferably is metal which may be applied by sputtering or vapor coating nickel, aluminum or indium tin oxide to a thickness of approximately 50 to 500 Angstroms (5 to 50 nanometers), preferably covered by a transparent protective layer as in the Yenni patents. Most figures of the Mueller drawing show an A-shape container with two open ends. A plug across each of the open ends provides a protective spacing between the contents and any external electrostatic discharges and also keeps the contents from falling out.
It has been suggested that a transparent box can be made from a box blank of a thermoplastic resin sheet which has been formed with flexible fold lines. See U.S. Pat. Nos. 4,064,206; 4,179,252; and 4,348,449 (all Seufert), the later two being divisions of the first. Seufert creates the scores or fold lines by pressing the sheet with a forming tool to a depth of at least 25% of the thickness of the sheet while maintaining a temperature between the softening temperature and the melting temperature of the thermoplastic sheet. The example of each of the Seufert patents employed "a highly transparent, plasticizer-free hard PVC material having a high impact resistance and K-value of 60". This was scored to 70% of its thickness of 0.25 millimeters and formed into box blanks that "had perfectly bendable edges and could be processed without difficulty first to flat folded boxes and then to unfolded boxes which were packed and closed on cardboard machines at a rate of approximately 180 pieces per minute.